1. Field of Invention
The invention relates generally to FM demodulators, and more particularly, to FM demodulators for reproducing FM signals recorded on magnetic tapes and disks.
2. Description of the Background Art
In the so-called 8 mm VTRs and optical video disk players, various types of signals such as video signals and audio signals are FM demodulated and recorded in mutual superimposition on the recording medium of magnetic tapes and optical disks. The frequency spectrum of a recorded signal on a magnetic tape of a 8 mm VTR is shown in FIG. 1, for example. It can be seen from FIG. 1 that a pilot signal, a low frequency range conversion color signal, an FM audio signal and an FM luminance signal constituted by four frequencies for tracking are frequency multiplexed, each using a predetermined frequency band. In the case where a recorded signal having such a frequency spectrum is reproduced and a desired signal, an FM audio signal for example, is to be separated, the lower limit of the FM luminance signal and the upper limit of the low frequency range conversion signal are so close to the band of the FM audio signal that the extraction of the FM audio signal by a band pass filter filter (hereinafter referred to as BPF) becomes critical.
Thus, for the general reproduction of FM signals not limited to a 8 mm VTR, the circuit shown in FIG. 2 is utilized. The cut off frequency of a BPF 1 is previously set to the band of a desired signal, for example, the frequency of an FM audio signal. An input FM signal from a reproduction head is provided to BPF 1, so that only the FM audio signal is extracted. This FM audio signal is provided to a limiter 2, where the AM component included in the FM audio signal is removed. Then, the FM audio signal is FM demodulated at an FM demodulator 3 to obtain a demodulated audio signal. The limiter 2 has the function to make the amplitude of the FM signal constant, while performing high gain amplification. The reason why this function is necessary will be explained hereinafter.
When a classical FM demodulator such as of a Foster-Seeley type is used for FM demodulation, the demodulator itself will operate as an FM demodulator as well as an AM detector. Therefore, the AM component will directly be accepted as a noise. When an FM demodulator using a digital circuit, such as a pulse count type FM demodulator is used, the necessity of digitizing the input FM signal occurs. Therefore, it is customary to use a limiter in an FM demodulator.
However, to interpose a limiter 2 between BPF 1 and FM demodulator 3, as shown in FIG. 2, interferes with the function of BPF 1. BPF 1 has the function to distinguish the objective signal from other signals and extracts the objective signal. In other words, BPF 1 has the function to change the amplitude depending on the frequency. On the contrary, limiter 2 attempts to make the amplitude constant regardless of the frequency.
It is assumed that BPF 1 has the characteristic shown by the solid line L1 of FIG. 3. Due to the effect of limiter 2, the high amplitude component is suppressed, while the low amplitude component is enhanced. Consequently, the characteristic of the FM signal passing limiter 2, i.e., the characteristic of the effective BPF is such as shown in the broken line L2 of FIG. 3. Therefore, when the FM demodulation method of FIG. 2 is employed in a 8 mm VTR, it would be impossible to completely remove the upper and lower video signals, namely the FM luminance signal and the low frequency range color signal, resulting in buzz disturbance even if a superior characteristic BPF is used.
Recently, a PLL-FM demodulator employing a phase locked loop (PLL) circuit comprised of a phase comparator, a low pass filter (hereinafter referred to as LPF), and a voltage controlled oscillator (hereinafter referred to as VCO) is used, as disclosed in Japanese Patent Laying-Open No. 62-20486. Since a PLL circuit does not respond to the AM component in principle, a limiter is unnecessary. However, in practice, the PLL circuit will respond to the AM component to some extent, and the AM suppression ratio is about 30-40 dB. Particularly in 8 mm VTRs, an output difference between two heads is liable to occur, with the effect of the AM component appearing significantly in the low frequency range. Accordingly, an amplitude limiting means such as a limiter will be necessary, but this application of a limiter will generate the aforementioned problems.
A reproduction output circuit employing a gain controlling circuit for obtaining an FM signal with a constant amplitude is disclosed in Japanese Patent Laying-Open No. 63-58602. With this reproduction output circuit, the amplitude of an input FM signal is detected by envelope detection, and the gain in the gain controlling circuit is controlled using this detected result. Thus, a constant amplitude can be obtained.
However, since the frequency is not distinguished in envelope detection, the envelope detection AGC (automatic gain control) will attempt to make the amplitude constant irrespective of the frequency. Therefore, it can be said that the function of the envelope detection AGC interferes with the function of the BPF, though not to the extent in the event of a limiter.
The case where an interference wave v from a video signal is superimposed upon an FM audio signal s is considered, as shown in FIG. 4A. FIG. 4B shows the instance where an FM audio signal is envelope detected. When the gain of a gain controlling circuit is controlled by the signal of FIG. 4B, the amplitude of the FM audio signal provided from the gain controlling circuit is not constant due to the interference wave from the video signal, as shown in FIG. 4C.